The invention relates generally to chemical sensors, and more particularly to artificial olfactory sensors.
Odors are complex mixtures of chemical species, and so contain hundreds, if not thousands, of constituent molecules. The biological olfactory system is a remarkable sensor which has some very important characteristics. There are very many olfactory cells or odorant receptors, but there are not very many different types of olfactory cells. The characterization of a scent or odor is not through a specific receptor or a specific type of receptor but through the combined response of many of the receptors. In effect, the sensors respond broadly to a range or class of odors rather than to a specific one. This is the opposite to the ideal gas sensor, which responds to only one gas, and provides a unique output for a selective species. Identification of the odor is through pattern recognition in the olfactory bulb and the subsequent neural processing in the brain. In addition, the signal-processing system removes drift and may enhance the overall sensitivity of the system by as many as three orders of magnitude. The entire system is highly compact and consumes extremely low power.
Given that the human olfactory system has 100 million olfactory cells (50 million per nostril) and that each cell has ten or more cilia, each with an odor receptor, a person has a total of 1,000 million or 1 billion odor receptors. But since there are only about 1000 types of olfactory receptors, there are thus approximately 1 million identical odor receptors of each type. This high degree of redundancy could provide for an improved ratio of signal to noise.
The ability to sense odors, or identify chemical species by their odors, has a wide range of applications. It can be deployed in many major and diverse industries such as: foods (e.g., quality control for wine and coffee), cosmetics (e.g., perfume), safety (e.g., explosive detection for airlines and bacteria detection such as the E. coli), humanity (e.g., detection of landmines), medicine (e.g. detection of pneumonia and some other infections), automobile (e.g., exhaust detection for pollution and fuel efficiency), etc.
In the case of explosive detection, it is particularly important and timely because, unfortunately, almost all of the most advanced and promising detection systems have some limitations in terms of what can be detected and where they can be used. Detectors which use radiation such as X-ray and other energetic particles cannot be used on human beings. These systems are intended primarily for baggage inspection. Furthermore, these technologies have not adequately addressed non-explosive devices that deliver chemical agents and they certainly are too expensive for consumer types of applications (such as quality control for coffee and wine), not to mention that they are totally inadequate for such jobs.
Practical options of detecting dangerous substances such as explosives include the use of dogs or vapor/particle detection devices. The importance of the K-9 approach stems from the high sensitivity of the dog""s olfactory system and the relatively low probability of false alarms. However, there are many limitations in using canines, including short attention span, mood changes, vulnerability to illnesses such as flu, the need to be retrained in new locations, and the apparent difficulty in finding samples placed higher than five feet. Vapor/particle detectors typically depend on classical spectrometric techniques, which rely upon the identification of specific molecules, and are not as sophisticated as the olfactory senses of animals. Thus, a suitable artificial olfactory device would provide tremendous strides in the detection of explosives and the 110 million uncleared landmines around the world since such a system will not have the same limitations at those found in the use of canines. The detection requirements for counter-terrorism and mine sweeping are quite similar. In both situations, the detection systems should be sensitive, reliable, low cost, and fast.
There have been many attempts in the past to mimic the biological olfactory system. Most of them are based on existing gas-sensor technologies and have many drawbacks. Gas sensors made from SnO2 are typical of current technology, and several commercial xe2x80x9celectronic nosesxe2x80x9d have been based on SnO2 arrays. Platinum pellistor-type elements, similar to SnO2 sensors, require a high power consumption, which interferes with portability and low power operation.
Sensor arrays have also been made from conducting organic polymers. However, there are only a few classes of stable conducting polymers and at the present time the conducting polymers must be synthesized electrochemically, which tends to produce insoluble, intractable materials. Additional variations in the array elements have been limited to such things as varying the substituents on the polymer backbone.
The scope of conducting polymer-based sensors has recently been broadened through the use of a set of polymer blends that possess a common conducting element, polypyrrole, for signal transduction, and a variety of insulating organic polymers to achieve chemical diversity in the array. It is suspected that polypyrrole and the odor molecules incorporated into the conducting polymer act as dopants and thus produce changes in its electrical resistance. These devices function quite well, but the long-term stability of polypyrrole is of concern for practical implementation of such systems.
At the California Institute of Technology, an array of chemiresistors is used. The chemiresistors are formed of a mixture of a conducting element (e.g., carbon or polypyrrole) with a non-conducting polymer. Its detection limit currently is low but is still many orders of magnitude less sensitive than biological odor-detection systems. In addition, there are questions about the manufacturability of such sensors, since according to percolation theory, the optimal region of operation occurs at the high-gain section, which is extremely sensitive to the amount of carbon. A 1% change in the amount of carbon black can result in more than 6 orders of change in the magnitude of the resistance in that region of operation. On top of that, it also relies on a mechanism that might suffer drift or miscalibration if exposed to high concentrations, since if the swelling is great enough, the carbon grains might rearrange themselves when it shrinks back.
At Tufts University, an array of fiber-optic sensors is used as an artificial nose. The sensors contain spatially-separated photopolymers containing analyte-sensitive fluorescent indicators on an imaging fiber tip. One important advantage is that many different sensors can be built on the distal end. Though the size of each of the sensing regions is about 30 microns, thus allowing a closely-packed structure, the overall size of the sensing system is large, and more than offset the potentially small size of the fiber itself. The cited overall 60% correct prediction rate is clearly not acceptable. There is also a question on the possibility of long-term drifts of the sensitivities of these sensors, since the solvatochromic dyes are subject to continuous illumination and may react with the gases that are being sensed.
It is thus an objective to develop an artificial olfactory system to meet the above critical needs and challenges. If an artificial olfactory system can be modeled after biological systems, it has the potential to be as sensitive as its biological counterparts; as a result, one of the most important advantages is that it can be used on humans and thus could potentially detect suicide bombers. The system could also be useful for the detection of land mines, non-nitrated explosives, explosive liquids and incendiaries. The olfactory system would be capable of detecting chemicals of many kinds, provided that they have sufficient outgassing to be detected.
Specifically, it is desired to provide a low-cost, ultra-sensitive, highly miniaturized (concealable), battery-powered, electronic chemical-sensing system, that is, an artificial olfactory system that would cost about a thousand dollars or less in a package about a few cubic inches (excluding the battery) with a detection limit of 100 ppt or less without pre-concentration or less than 1 ppt with pre-concentration.
The invention is an artificial olfactory system which includes materials, methods, apparatus, and intelligent processors for the detection of odorant molecules or biological agents or any other chemical or biological species in a surrounding fluid, e.g., air. The apparatus includes a sensor having first and second conductive elements (e.g., electrical contacts) electrically coupled to a chemically or biologically sensitive acoustic device (such as a quartz crystal microbalance(QCM), surface acoustic wave device (SAW), or micro-machined resonator) which provide an electrical path between the pair of conductive elements. The acoustic device is a resonator coated with aerogel or other equivalent material (such as nanotubes, porous carbons, zeolites) or with a material formed by certain methods (such as micro-machining) to expand the detecting surface area of the uncoated resonator by a large amount (by a factor of 1000 or better). The aerogel or other surface area increasing material is coated with a polymer or equivalent material that can be tuned for the attachment of the odorant molecules or biological agents or other chemical or biological species. An indirect means to detect a biological agent is to have a reactive substance, such as appropriate protein, that reacts with the biological agent resulting in the generation of odorant molecules that can then be detected by the invention. In use, the resonator provides a difference in resonating frequencies when contacted with a fluid containing odorant molecules or biological agents at different concentrations. The artificial olfactory system uses a plurality of these sensors, each with a different response to particular odorant molecules or biological agents.
The resonating frequency is typically in the megahertz range. Variability in sensitivity from sensor to sensor is conveniently provided by qualitatively or quantitatively varying the aerogel or its equivalent materials and the polymer or its equivalent materials. For example, in one embodiment, the aerogel material in each sensor is held constant (e.g., certain type of aerogel known as xerogel) while the organic polymer varies from sensor to sensor. Arrays of these sensors are constructed with at least two sensors having different chemically sensitive polymers providing dissimilar differences in resonating frequencies. The artificial olfactory system is constructed by using such an array in conjunction with a frequency measuring device electrically connected to the conductive contacts of each sensor. The artificial olfactory systems also incorporate a variety of additional components including signal processing hardware and software to determine the identity, etc. Methods of making and using the disclosed sensors, arrays, and artificial olfactory systems are also provided.